The present invention concerns a process for preparing cellulose pulp. According to such a process a fiber-based-starting material is delignified in cooking liquor containing cooking chemicals to yield pulp, and the obtained pulp is bleached if desired.
This invention also relates to an apparatus for the pretreatment of raw material for pulping processes to enhance delignifiability.
Recent development in the pulp industry has resulted in ever greater and more expensive investments. The most modem pulp mills already produce over 2000 metric tons of pulp per day. The cost of such mills is about FIM 4-5 billion, and there are not many customers in the world who can take such an enormous technological and economic risk. In the future, the ever increasing environmental pressures will introduce new risk factors as well. Logistics, the transportation of raw materials and products worldwide, constitutes both a cost factor and an environmental risk. One of the disadvantages of the big mills is that they require quite homogeneous raw material. Raw material with varying fibre properties cannot be fed into the cooking process without adverse effects on the pulp properties.
There are thousands of environmentally less friendly small pulp mills in the world, especially in Asia. The general development in the industry has not reached these areas, because it has not been regarded as profitable to develop environmentally friendly small-scale mills by the major suppliers of technology. However, in the recent years, the interest in these small mills facing closing down has increased. As regards the raw material base, they are more flexible than larger mills. Thus, almost 15% of all pulp is made from annual plants, non-wood fiber material, in such small scale pulp mills.
Annual plants have the advantage as raw material that they are easy to cook in a homogeneous manner. Contrary to this, it is typical of pulping processes based on the use of wood chips that the surface and the inner parts of the chips become treated differently. The surface zone is "overcooked" and the inside remains "raw". A similar phenomenon occurs if the starting material for pulping processes comprises a combination of annual and perennial plants, the annual ones being cooked with considerable ease in comparison to the perennial ones. The average quality achieved during grinding (refining) is a combination of fibers with different degrees of ripeness. In mechanical pulping processes, in which the raw material is subjected to mechanical impacts, the pulping effect achieved is more even than in the case of chemical pulping. For example, the grinding effect of a grinding stone on the surface layer of wood is equal to that imposed on the fibres of the inner layer during the preparation of ground wood pulp.
In order to cause the lignin glueing together the fibers in the wood chips to dissolve throughout during chemical pulping, it is necessary to cook the chips at an elevated temperature and pressure. Thus, the cooking installation with a pressure cooking vessel will be quite expensive, wherefore only the above-cited large mills (&gt;400 000 tons of pulp/year) are economically profitable using known techniques.
It would be ideal to arranger such conditions for chemical pulp production that each fiber in the wood material receives identical treatment. This is well known, but no suitable method in which the fiber structure is not broken down too extensively has been discovered.
The object of this invention is thus to provide an entirely novel approach to the preparation of pulp by a chemical pulping process. More specifically, it is the object of this invention to provide a method which causes the pulping process to become homogeneous in such a way that the strength properties of the fibers are retained. This being the case, so called wood material of lesser value (such as alder, aspen and mixed tropical hardwood) can be used for the preparation of usable pulp.
This invention is based on the principle that wood chips or similar lignocellulosic raw material is precrushed to cause its structure to become open. The precrushing according to this invention is performed in a pulsating manner, with the aid of pressure shocks in the cooking liquour, which causes the fiber structure of the raw material to become efficiently impregnated with the cooking liquour due to the alternation of elevated and reduced pressure action during the crushing stage. The fibers start becoming cooked already in connection with the pretreatment, and the invention provides a three-stage cooking process, in which the raw material separates into fibers during all the three stages, presoaking, crushing and cooking. By using the same liquor (possibly diluted with water during the pretreatment stage) it is also possible to facilitate the handling and regeneration of liquids in the process.
Some techniques for precrushing wood chips are previously known in the art. These have been described in the following patent specifications: FR 2 276 420, FI 70937, FI 77699, FI 94968 and SE 461 796. In prior art apparatuses, the chips are usually pressed between two rolls in order to cause the chips to become crushed or to facilitate impregnation by liquids. An apparatus consisting of two pairs of rolls positioned on top of each other is described in Fl Patent Specification No. 94968, in which apparatus an "agressive" profile is formed on the surface of the rolls. This kind of serrated profile causes sharp, cutting surfaces that cut fibers and weaken the strength properties of the raw material to be treated.